1. Field of the Invention
The field of the invention relates generally to transmissions, and more particularly the invention relates to continuously variable transmissions.
2. Description of the Related Art
In order to provide a continuously variable transmission, various traction roller transmissions in which power is transmitted through traction rollers supported in a housing between torque input and output discs have been developed. In such transmissions, the traction rollers are mounted on support structures which, when pivoted, cause the engagement of traction rollers with the torque discs in circles of varying diameters depending on the desired transmission ratio.
However, the success of these traditional solutions has been limited. For example, in one solution, a driving hub for a vehicle with a variable adjustable transmission ratio is disclosed. This method teaches the use of two iris plates, one on each side of the traction rollers, to tilt the axis of rotation of each of the rollers. However, the use of iris plates can be very complicated due to the large number of parts that are required to adjust the iris plates during transmission shifting. Another difficulty with this transmission is that it has a guide ring that is configured to be predominantly stationary in relation to each of the rollers. Since the guide ring is stationary, shifting the axis of rotation of each of the traction rollers is difficult.
One improvement over this earlier design includes a shaft about which an input disc and an output disc rotate. The input disc and output disc are both mounted on the shaft and contact a plurality of balls disposed equidistantly and radially about the shaft. The balls are in frictional contact with both discs and transmit power from the input disc to the output disc. An idler located concentrically over the shaft and between the balls applies a force to keep the balls separate so as to make frictional contact against the input disc and output disc. A key limitation of this design is the absence of means for generating and adequately controlling the axial force acting as normal contact force to keep the input disc and output disc in sufficient frictional contact against the balls as the speed ratio of the transmission changes. Due to the fact that rolling traction continuously variable transmissions require more axial force at low speed to prevent the driving and driven rotating members from slipping on the speed changing friction balls, excessive force is applied in high speed and at a 1:1 ratio, when the input and output speeds are equal. This excessive axial force lowers efficiency and causes the transmission to fail significantly faster than if the proper amount of force was applied for any particular gear ratio. The excessive force also makes it more difficult to shift the transmission.
Therefore, there is a need for a continuously variable transmission with an improved axial load generating system that changes the force produced as a function of the transmission ratio.